In an electric power generating station, a process control unit, or controller, compares values of a process characteristic, or controlled variable, with a target value, or setpoint, to determine whether the process is operating within acceptable bounds. If the controller determines that the controlled variable is outside normal bounds, it can initiate corrective action itself by adjusting a parameter, or manipulated variable, of the process.
A control system as described above is referred to as a feedback system because the value of the manipulated variable is generated in response to the value of the controlled variable. Thus, the controlled variable is fed back into the controller to become one of the inputs to the controller.
In principle, a feedback system can compensate for external disturbances to the extent those disturbances affect the value of the controlled variable which is fed back to the controller. For example, in the case of a heated room, opening the window (an external disturbance) causes a drop in temperature (the controlled variable) which in turn is sensed by the thermostat (the controller). In response, the thermostat increases current flow (manipulated variable) in an effort to re-establish the desired temperature (setpoint). In such a system, the controller responds to the external disturbance indirectly by monitoring any change in the value of the controlled variable.
In many control systems, there can be a delay between the occurrence of the external disturbance and the controlled variable's response to that disturbance. As a result of this delay, the controller may not initiate corrective action promptly. It is possible, in such cases, for the delay to be so long that by the time the controller initiates corrective action, significant damage will have occurred. In such cases, it may be advantageous to sense the disturbance directly rather than through its effect on the controlled variable. If in the heated room of the above example, it were critical that the temperature be held constant, it would be advantageous to provide a signal from the window directly to the thermostat. This connection, from the external disturbance to the controller, is referred to as the feedforward loop of the control system.
A feedforward loop is advantageous because it allows the controller to anticipate the effect of the disturbance on the controlled variable and to thereby compensate for this effect before the effect actually occurs. However, before the controller can do so, it must determine what the effect of the disturbance will be on the control system. In the context of the above example, a thermostat which recognizes that a window has been opened will not necessarily know how much additional current to supply to the heating system. We refer to this information on the effect of a disturbance on the control system as the calibration data.
In the prior art, intervention by a service engineer or other field service personnel is typically required to provide the necessary calibration data to the multivariate controller. Because of the complexity inherent in multivariate control system, reliance on a service engineer can result in error.
What is therefore desirable in the art is a convenient method and system for bypassing human intervention and automatically providing calibration data to the multivariate controller.